The present invention relates to a method and a device for aligning sheets. More specifically the invention relates to a method and a device for aligning thermal-sensitive sheets to be used in a thermal printer.
Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy.
In thermography three approaches are known:
1. Direct thermal formation of a visible image pattern by image-wise heating of a recording material containing matter that by chemical or physical process changes color or optical density.
2. Image-wise transfer of an ingredient necessary for the chemical or physical process bringing about changes in color or optical density to a receptor element containing other of the ingredients necessary for said chemical or physical process followed by uniform heating to bring about said changes in color or optical density.
3. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a colored species from an imagewise heated donor element onto a receptor element.
Thermographic materials of type 1 can be rendered photothermographic by incorporating a photosensitive agent which after exposure to UV, visible or IR light, e.g. by means of a laser, is capable of catalyzing or participating in a thermographic process bringing about changes in color or optical density.
A survey of direct thermal imaging methods is given in the book xe2x80x9cImaging systemsxe2x80x9d by Kurt I. Jacobson-Ralph E. Jacobson, The Focal Pressxe2x80x94London and New York (1976), Chapter VII under the heading xe2x80x9c7.1 Thermographyxe2x80x9d.
Common thermal printers that do no use a laser light source comprise a rotatable drum and an elongate thermal head which is spring-biased towards the drum to firmly line-wise contact a heat-sensitive material which is passed between the head and the drum. The thermal head includes a plurality of heating elements. The image-wise heating of a sheet is performed on a line by line basis, with the heating elements geometrically juxtaposed along each other in a bead-like row running parallel to the axis of the drum. Each of these elements is capable of being energized by heating pulses, the energy of which is controlled in accordance with the required density of the corresponding picture element. The sheet is advanced between the head and the drum by frictional contact of its rear side with the drum.
Patent application EP-A-0 846 565 discloses such a thermal printer having a thermal head.
The images that are printed on such a thermal printer are often used for diagnostic purposes, medical diagnosis in particular. Customarily such images for medical diagnosis are printed on a transparent support. Examples of such images are echograms, CT scans, NMR images. These images are negative-type images, which means that their background is substantially black, the image details having lesser optical densities. FIG. 1 shows two sheets 10 that bear images that are printed by a thermal printer having a thermal head. The image areas E are substantially black and the margins A, B, C and D are transparent. The image cannot be printed on the sheet up to the edge since otherwise the thermal sensitive layer of the sheet would be squeezed at the edge due to the pressure between head and drum, which would soil the thermal head and the transport rollers.
These images are viewed on a light box for diagnosis. On the light box, the images can be positioned so that the transparent margins B are outside of the illuminated area while black screens can be moved in the light box, like curtains, so that they cover the margins C and D. However, if two sheets 10 are positioned alongside each other, as shown in FIG. 1, a transparent area between the two image areas E remains.
Radiologists are unfamiliar with such a transparent area, which does not exist in conventional AgX X-ray images. Moreover, a large transparent area has a dazzling effect. FIG. 1 shows two mammographic images. In mammography, it is customary to view the images of the right and of the left breast on a light box, positioned with respect to each other as shown in FIG. 1 (reference sign 15 in FIG. 1 indicates the contours of the breasts). Both sheets 10 are pushed against each other so that no space is left between them (for clarity, in FIG. 1 an open space is shown between the sheets 10). Thus, a transparent area of twice margin A remains between the images. Up till now, such mammographic images did not have a transparent margin, because they were made e.g. by a conventional AgX apparatus or in a photothermographic printer wherein the laser can expose the sheet up to its edges.
It would be advantageous to print such mammographic images by means of a printer with a thermal head, since this is less expensive than using a photothermographic printer. However, when printed by a conventional printer with a thermal head, on a light box the transparent area between the two images is disturbing.
It is therefore an object of the invention to provide a thermal printer having a thermal head that can print mammographic images that are suitable for diagnosis on a light box.
It is a further object of the invention to provide a method that allows obtaining, by means of a thermal printer having a thermal head, mammographic images that are suitable for diagnosis on a light box.
The above-mentioned objects are realised by a thermal printer including a device as claimed in claim 1 and claim 5 and by a thermal printer performing a method as claimed in claim 17 and claim 19. The dependent claims set out preferred embodiments of the invention.
A sheet 10 having a substantially straight edge 11, as shown in FIG. 1, is accurately aligned in accordance with the invention. An image can then be printed on the sheet leaving only a small margin A, of e.g. 1.1 mm, between the image area E and the substantially straight edge 11. A transparent area of twice such a small margin A, between two image areas E, is not disturbing when viewed on a light box. Because of the accurate alignment, the margin A has a nearly constant width so that there is no risk of the image area E coming too close to the sheet edge 11, which would result in soiling the thermal head as mentioned above. One margin, margin A in FIG. 1, has a small width; the other margins, margins B, C and D in FIG. 1, may have a larger width.
In a preferred embodiment of the invention, shown in FIG. 2, the substantially straight edge 11 of sheet 10 is aligned with respect to an alignment axis 25 that is substantially perpendicular to the axis 45 of the drum of the thermal printer. Sheet 10 as shown in FIG. 1 may be a substantially rectangular sheet having the standard dimensions of 10xe2x80x3xc3x9712xe2x80x3. The image is then printed line-wise with the image lines substantially perpendicular to edge 11, i.e. substantially parallel to edge 12. Preferably, edge 12 is the short, 10xe2x80x3, sheet edge and edge 11 is the long, 12xe2x80x3, sheet edge. An advantage of this embodiment is that a shorter and hence less expensive thermal head may be used than if the printed image lines would be substantially parallel to the longer sheet edge 11.
In this text, a xe2x80x9csubstantially straight edgexe2x80x9d of a sheet is defined as follows. LS is the straight line segment that is the least squares fit of the edge. An edge is substantially straight if, for all points PT in line segments S belonging to the edge, so that the total length of the line segments S is at least 80% of the length of the edge and preferably at least 90% of the length of the edge, the distance d between PT and LS is d less than 5 mm, preferably d less than 3 mm, more preferably d less than 1 mm and most preferably d less than 0.5 mm. The distance between points PT and straight least squares segment LS may be larger over portions of the edge (of relative length 20% or 10%) to allow for e.g. notches which are quite customary in medical film sheets.
A first line L1 is xe2x80x9csubstantially parallelxe2x80x9d to a second line L2 if, when L1 is the line parallel to L1 through an arbitrary point O taken as origin and L2* is the line parallel to L2 through O, the smallest angle xcex1 between L1* and L2* is a  less than 15xc2x0, preferably xcex1 less than 10xc2x0, more preferably xcex1 less than 5xc2x0.
A first line L1 is xe2x80x9csubstantially perpendicularxe2x80x9d to a second line L2 if, when L1* is the line parallel to L1 through an arbitrary point O taken as origin and L2 is the line parallel to L2 through O, the smallest angle xcex2 between L1* and L2* is xcex2 greater than 75xc2x0, preferably xcex2 greater than 80xc2x0, more preferably xcex2 greater than 85xc2x0.
A line is xe2x80x9csubstantially verticalxe2x80x9d if it is substantially parallel to a vertical line; a vertical line has the same direction as the force of gravity.
A xe2x80x9csubstantially horizontalxe2x80x9d line is substantially perpendicular to a vertical line.
A sheet is xe2x80x9csubstantially rectangularxe2x80x9d if it has four substantially straight edges and if the adjoining edges are substantially perpendicular to each other, as defined above.
Further advantages and embodiments of the present invention will become apparent from the following description and drawings.